The thermal budget defines the maximum allowable temperature rise of a heat source when operating at its maximum thermal design power (TDP). Engineers calculate this by subtracting the maximum ambient operating temperature (degrees C) of the device from the maximum allowable temperature of the heat source, usually given as Tcase Max for lidded ICs.
To determine the maximum thermal resistance of a heat sink, divide the thermal budget by the TDP.
Heat Sink Design and Thermal Resistance Components
Heat sink design involves managing multiple layers of thermal resistance, which directly impact overall thermal performance. These resistances occur at various points in the heat transfer path. Its components are shown on the right in the illustration below while the heat transfer mechanism associated with those resistances is shown on the left.
Elements of Heat Sink Thermal Resistance Network
Understanding the thermal budget and thermal resistance targets is crucial during both the early stages and optimization phases of product development. These figures help engineers determine the likely classification of the thermal solution best suited for the application: solid metal heat sink (0.5-2.0 C/W), heat pipe based heat sink (0.1-0.5 C/W), or pumped liquid systems (0.01-0.1 C/W).
Steps for Optimized Heat Sink Design
- Calculate the thermal budget and maximum target thermal resistance to establish design constraints.
- Use online tools like a heat sink size calculator to estimate heat sink volume and dimensions based on initial requirements.
- Run simulations or tests using a heat sink performance calculator to analyze ΔT across thermal resistance categories. Note: This calculator shows results for both solid metal and vapor chamber based heat sinks. The performance of heat pipe based designs will be somewhere in between the two.
- Optimize critical areas, focusing on reducing dominant resistances.
Optimizing the Heat Sink to Reduce Thermal Resistance
Engineers analyzing heat sink temperature rise (Delta-T) with the calculator can identify areas for improvement by pinpointing which components or mechanisms, such as TIM, base-to-fin conduction, or convection, contribute most to temperature deltas. This allows targeted optimizations like better materials, smoother surfaces, improved airflow, or enhanced fin designs.
Temperature Rise in a Thermal Network
The chart above shows that the two biggest areas for improvement are Air and Fin-to-Air elements of the thermal network. Temperature rise here is driven by convection rather than conduction.
High ΔT due to convection: Optimize the heat sink by increasing fin area and/or airflow.
High ΔT due to conduction: Optimize by using more thermally conductive materials, improve surface smoothness, or increase clamping pressure.
